Diels-Alder polysulfones as dielectric materials: Computational guidance & synthesis

Article abstract of DOI:10.1016/j.polymer.2014.06.041

Herein, we describe the synthesis and characterization of polymers formed by the Diels-Alder (DA) reaction between various difurans and divinylsulfone, and evaluate their dielectric properties. These syntheses were planned with computational support a priori in the form of HOMO/LUMO calculations for the dienes and dienophiles, with the calculated ΔE for the DA reactions ranging between 9.33 and 9.42 eV. We describe the structure-property relationship observed when changing an atom in the linking unit between two furan rings (-CH2-, -O-, -S-, -NH-) with respect to the dielectric constant and loss tangent. Dielectric constants for the polymers range between 4.96 and 5.98, with dielectric loss tangents ranging from 0.4 to 0.9% at 1 kHz and room temperature. Bandgaps of the polymers are elucidated with UV/Visible spectroscopy, and range from 2.15 to 2.61 eV. The retro DA onset is determined using three analytical methods: thermogravimetric analysis, dynamic scanning calorimetry, and gas chromatography/mass spectrometry polymer desorption spectroscopy, and is determined to be over 125 °C in all cases.

Full text of DOI:10.1016/j.polymer.2014.06.041

Accepted Manuscript
Diels-Alder Polysulfones as Dielectric Materials: Computational Guidance & Synthesis
Robert G. Lorenzini, Jordan A. Greco, Robert R. Birge, Gregory A. Sotzing
PII:
S0032-3861(14)00535-7
10.1016/j.polymer.2014.06.041
JPOL 17060
DOI:
Reference:
To appear in: Polymer
Received Date: 24 April 2014
Revised Date: 11 June 2014
Accepted Date: 13 June 2014
Please cite this article as: Lorenzini RG, Greco JA, Birge RR, Sotzing GA, Diels-Alder Polysulfones
as Dielectric Materials: Computational Guidance & Synthesis, Polymer (2014), doi: 10.1016/
j.polymer.2014.06.041.
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ACCEPTED MANUSCRIPT

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Diels-Alder Polysulfones as Dielectric Materials: Computational Guidance
&
Synthesis
§,‡
§
§
§,‡,*
Robert G. Lorenzini , Jordan A. Greco , Robert R. Birge , Gregory A. Sotzing
§
‡
Department of Chemistry & The Polymer Program, University of Connecticut,
55 North Eagleville Road, Storrs, CT 06269, United States
*
Correspondence to: Gregory A. Sotzing (E-mail: g.sotzing@uconn.edu)
Keywords: Dielectric properties, Structure-property relations, Polysulfones
Abstract
Herein, we describe the synthesis and characterization of polymers formed by
the Diels-Alder (DA) reaction between various difurans and divinylsulfone, and
evaluate their dielectric properties. These syntheses were planned with
computational support a priori in the form of HOMO/LUMO calculations for the
dienes and dienophiles, with the calculated ∆E for the DA reactions ranging
between 9.33-9.42 eV. We describe the structure-property relationship observed
when changing an atom in the linking unit between two furan rings (-CH2-, -O-, -
S-, -NH-) with respect to the dielectric constant and loss tangent. Dielectric
constants for the polymers range between 4.96-5.98, with dielectric loss tangents
ranging from 0.4-0.9% at 1kHz and room temperature. Bandgaps of the polymers
are elucidated with UV/Visible spectroscopy, and range from 2.15-2.61 eV. The
retro DA onset is determined using three analytical methods: thermogravimetric
analysis, dynamic scanning calorimetry, and gas chromatography/mass
spectrometry polymer desorption spectroscopy, and is determined to be over
125 °C in all cases.

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Introduction
The need for high energy density capacitors is driven by their importance in
1
various cutting-edge technologies such as medical equipment, electromagnetic
2
3
4
weaponry, hybrid electric cars, and controlled nuclear fusion. The current
industrial standard, biaxially-oriented polypropylene (BOPP), has a dielectric
-
4
constant of ~2.2 across a broad frequency range, low dielectric loss (~1x10 )
5
and a breakdown strength of ~700MV/m. Other efforts to achieve high energy
6,7
density in dielectric materials are focused on PVDF composites, aromatic
8,9
10
polyureas and polythioureas, hybrid organic/inorganic composites, and the
11,12
13
functionalization of polypropylene,
among others. In line with other research
14,15,16
in our group,
inclusion of electronegative atoms into the polymer backbones. The Diels-Alder
we seek to increase the dielectric constant of materials by the
17
(DA) reaction involving vinyl sulfones and sulfonates are widely known in the
18,19, 20, 21, 22, 23, 24
literature for their monofunctional adducts,
but to our knowledge,
no groups have used them as difunctional dienophiles for inclusion in the
backbone of a DA polymer. DA reactions involving furans are a facile means to
increase the number of oxygen atoms in a polymer. In addition, the DA reaction
does not form any side products inherent to the mechanism, and therefore does
not need post-synthesis purification. Designing a sulfone group into the
backbone of other polymers is known to generate high refractive indices, and
25
therefore a high electronic contribution to the dielectric constant. The difuran-
divinylsulfone system (Figure 1) is especially attractive as the two liquid
monomers can form polymer films by simply mixing them and heating in an oven
at 90°C for 24h, obviating the need for complex fil m processing methods. This
work evaluates the potential of these polymers to serve as high dielectric
constant, high bandgap dielectric materials. Elucidating the HOMO/LUMO gap
between the dienes and dienophiles using commercially available computational
software screens the feasibility of our syntheses before we begin work in the
laboratory. We test the dielectric properties of the synthesized polymers at
multiple temperatures using a time-domain dielectric spectrometer equipped with

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Figure 1 – Schematic of monomers and Diels-Alder polysulfones.
an oven. One potential difficulty with polymers formed through the DA
mechanism is the possibility of the reverse, or the retro Diels-Alder (rDA) reaction.
This temperature is of crucial significance, as it is the absolute maximum
temperature the material could be exposed to without degradation. To probe the
onset of the rDA reaction, we employ three analytical methods, each relying on
different principles.
Experimental
Theoretical Methods
All ground state geometries of divinylsulfone and the various difurans were
26
obtained using a Becke, 3-parameter, Lee-Yang-Parr (B3LYP) hybrid functional
27,28
and a 6-311++G(d,2p) basis set.
Following the geometry optimizations, the
self-consistent field (SCF) theory was implemented by using the restricted
29
Hartree-Fock (RHF) procedures and a 6-311++G(d,2p) basis set in order to
assign the energies of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) for each molecule. All calculations
30
were implemented in Gaussian 09.

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Monomer Synthesis
,3-Bis(2-furyl)propane (difuran A) was synthesized according to the literature
31
1
by deprotonating two equivalents of furan with n-butyllithium, followed by the
addition of one equivalent of dibromopropane and vacuum distillation from the
1
monosubstituted side product, isolated yield: 42%. H NMR: 7.33 (2H, s), 6.31
(2H, m), 6.04 (2H, m), 2.70 (4H, t) 2.02 (2H, m)
1
The syntheses and H NMR characterization of difuran monomers B – D were
15
described in a previous publication by this group.
Divinylsulfone was purchased from Sigma Aldrich and was used as received.
Polymer Synthesis
Equimolar amounts of divinylsulfone and a difuran were combined in a 20mL
scintillation vial and were stirred by aspirating the liquids in and out of a pipette
copiously. The mixed monomers were cast onto a stainless steel shim stock
sitting in an oven, and were covered with a petri dish. The oven was set at 90°C
and the reaction was allowed to proceed for 24h.
Thermal Characterization, rDA
Thermogravimetric analysis (TGA) was conducted in oxygen using a TA
o
Instruments TGA Q500, heating rate = 10 C/min. Differential scanning
calorimetry (DSC) was conducted in nitrogen using a TA Instruments DSC Q100,
o
o
heating rate = 10 C/min. The samples were first heated to 100 C to clear the
o
thermal history, then were cooled to -25 C. The presented data were obtained on
the second scan. These two techniques provide information regarding the rDA
temperature. Using TGA, the temperature at 2% weight loss verified the rDA as
volatile monomers evaporate away. With DSC, the onset of the rDA (in the form
of the onset of the simultaneous rDA and endo-exo isomerization) manifests itself
as a broad exothermic peak. This exothermic peak persists if a second heating
run is conducted, albeit shifted to a higher temperature, as the rDA remains but
32
the endo-exo isomerization has already occurred.

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Polymer Desorption Gas Chromatography/Mass Spectrometry
The rDA was explored in a third way using a polymer desorption GC/MS method
14, 15
previously reported by our group.
Briefly, a 1mg sample of polymer is added
to a small glass test tube, after which it is placed in a custom-made device. The
tube was dropped into the heating block of the GC/MS and desorbed materials
proceed into the traditional GC/MS architecture. Experiments were conducted
between 100-170 °C in 10 °C intervals.
Gel Permeation Chromatography
Gel permeation chromatography was done using a Waters 515 HPLC pump,
Waters 2414 refractive index detector, and two mixed bed Jordi Gel DVB
columns. The polymer samples were dissolved in dimethylacetamide, and the
results were quantitated with polystyrene standards.
Time Domain Dielectric Spectroscopy
Time-domain dielectric spectroscopy was performed at the UConn Electrical
Insulation Research Center using silicone-guarded rubber electrodes. Dielectric
spectra and loss tangent measurements were taken at room temperature, 50, 75
and 100 °C between 10Hz and 10kHz.
UV/Vis Spectroscopy, Nuclear Magnetic Resonance Spectroscopy
UV/Visible spectroscopy was conducted on a Varian Cary 5000 between 200-
800nm. Samples were prepared as follows: polymers were dissolved in
chloroform, cast onto quartz slides, and dried in a vacuum oven at 50 °C for 2h.
1
H NMR spectroscopy was conducted on a Bruker DMX-500, and all chemical
3
shifts are relative to the solvent residual peak of CDCl (� =7.26 ppm).

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Results & Discussion
Computational HOMO/LUMO Values
We theoretically examined the validity of the proposed DA polymerization
reactions by carrying out a series of molecular orbital (MO) calculations with
three goals. First, the favorability of the reaction is examined by calculating the
properties of the relevant HOMOs and LUMOs of divinylsulfone and the difuran
starting materials. Second, the MOs are examined to determine the extent to
which the reactions obey Woodward-Hoffmann selection rules of the [4+2]
cycloaddition. Finally, the relative favorability of the four different difuran starting
materials is investigated.
Figure 2 demonstrates the circumstance in which a normal electron demand
mechanism is used to describe the DA polymerization mechanism. This case
involves the HOMO of the diene overlapping with the LUMO of the dienophile in
order to cyclize, and the band gap (∆E) is typically on the order of 0 – 12 eV.
Table 1 lists the calculated energies of the frontier MOs for the monomers. The
calculated ∆E values for the four difurans with divinylsulfone range between 9.33
and 9.42 eV, which is below a restrictive energy gap for the reaction. For
comparison, ∆E for the well-known DA reaction between butadiene and ethylene
33
is 10.6eV. The ∆E values for the normal electron demand mechanism (Table 1)
are all lower than the respective inverse electron demand values (data not
shown), in which the HOMO of the dieneophile overlaps with the LUMO of the
diene. Therefore, the normal electron demand scheme will be the only one
considered here. The relative ∆E values calculated for the four difuran
monomers correspond to the following trend: B > D > A > C. These data suggest
that greater electronegativity at the R position of the difurans removes electron
density from the HOMO, thereby decreasing the HOMO energy and increasing
∆
E.

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Figure 2 – Normal electron demand Diels-Alder reaction between difurans and
divinylsulfone.
Figure 2 also provides an illustrative example of the frontier MOs for difuran A
and divinylsulfone, as predicted by the calculations. The orbitals suggest
Woodward-Hoffmann selection rules for a [4+2] cycloaddition, wherein the MO
phases of the bonding π electrons are localized over the double bonds of the
furan and are capable of overlapping with the virtual MO localized on the vinyl
groups of divinylsulfone. Although difuran A is the only difuran molecule shown,
the HOMO and LUMO character is qualitatively similar to that observed in Figure
2
yielding comparable Woodward-Hoffmann selectivity.
Molecule
divinylsulfone
difuran A
EHOMO (eV)
-11.3344
-8.5177
-8.5928
-8.4970
ELUMO (eV)
0.8289
1.0093
1.0441
0.9962
1.0305
∆E (eV)
N/A
9.3466
9.4217
9.3259
9.3697
difuran B
difuran C
difuran D
-8.5408
Table 1 – Monomer HOMO, LUMO, and ∆E values.
The monomeric example provides valuable insight into the electronic features of
our starting materials. However, we also investigate the case that resembles the

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dimeric form; in Figure 3, one vinyl group of divinylsulfone has undergone a DA
reaction with one of the furan groups of difuran A. The result is a molecule with
the vinylsulfone group on one end and a furan group on the other, thereby
mimicking the reactivity of a higher polymer. Table 2 demonstrates that the
energy gap between the HOMO and LUMO for each of the four furan/sulfone
dimers is the same order of magnitude as the monomeric materials. Furthermore,
the HOMO and LUMO for the difurans are predicted to localize on the furan and
vinyl groups of the molecules, respectively, and adhere to similar phase
character that indicates Woodward-Hoffmann guidelines for the DA reaction. In
this case, difuran C has the highest ∆E value, which is observed due to
localization of the HOMO on the sulfur atom, in addition to the furan moiety. This
interaction at the R position is unique to difuran C, and the other three molecules
follow the electronegativity trend suggested by the results in Table 1. Despite
these small differences, the computational results predict the successful DA
polymerization with all difuran monomers/divinylsulfone.
Figure 3 – Dimeric HOMO and LUMO as a model for polymerization.

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Molecule
difuran A
difuran B
difuran C
difuran D
EHOMO (eV)
-8.5874
-8.6837
-8.7710
-8.6315
ELUMO (eV)
0.7622
∆E (eV)
9.3496
9.4663
9.5768
9.3928
0.7826
0.8057
0.7614
Table 2 – Dimeric HOMO, LUMO, and ∆E values.
Thermal Data, rDA, NMR
Figures 4 & 5 show the TGA and DSC data, respectively. Table 3 shows the
combined thermal data for the polymers, including the T and rDA temperatures
g
elucidated through the aforementioned methods. The rDA temperatures from the
three different methods are in good agreement. rDA temperatures are
ubiquitously above 125 °C , and therefore will not ne gatively impact their potential
application as dielectric materials. Of concern, however, is the fact that the T is
g
observed to be within the application operating range for polymers B & C. This
Figure 4 – TGA traces of the synthesized polymers with the 2% weight loss points
highlighted.

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Figure 5 – DSC traces of the synthesized polymers; the y-axis is offset for clarity.
feature is likely responsible for the sudden increase observed in dielectric loss
tangents at elevated temperatures. Because of the different DA adduct
combinations during polymerization (endo-endo, endo-exo, and exo-exo) the
NMR spectra for the polymers were difficult to quantify without synthesizing
model compounds; NMR was used to confirm polymerization based on the lack
of furanyl protons. Most conspicuously, the furthest upfield aromatic peaks of the
15
furan ring present in the starting materials ranging between 7.45-7.33 ppm
were absent in the spectra obtained from the polymer samples.
Polymer
Tg, °C
rDA by
GC/MS, °C
150
rDA by
TGA, °C
169
rDA by
DSC, °C
128
Mw, g/mol Mn, g/mol
PDI
A
B
C
D
95
47
68
38,200
54,400
47,000
46,300
9,500
12,800
12,600
8,900
4.02
4.25
3.73
5.20
160
170
160
176
197
187
142
143
179
113

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Table 3 – Thermal data and rDA temperatures
Dielectric Spectra & Trends in Dielectric Constant
The dielectric spectra and loss tangents as a function of temperature are
displayed in the Supporting Information, and the overlaid spectra at room
temperature are shown in Figure 6. The observed trend in dielectric constant is:
A > D > B > C. We rationalize this trend by considering the local electronegativity
differences between the DA adduct and the spacer units contributed by the
difurans. For example, the alkyl substituent in polymer A makes the spacer unit
have a larger difference in electronegativity overall (considering the bridging
oxygen in the DA adduct) than an ether substituent, as in polymer B. In all cases,
the dielectric constants of these polymers exceed that of BOPP (~2.2), and have
dielectric loss tangents below 1.5%.
6.50
6.00
5.50
5.00
4.50
A
B
C
D
1
0
100
1000
10000
Frequency, Hz

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0
0
0
.025
A
B
C
D
0.02
.015
0.01
.005
0
1
0
100
1000
10000
Frequency, Hz
Figure 6 – Dielectric spectra (top) and dielectric loss tangents (bottom) of the
synthesized polymers at room temperature.
Bandgap Measurements
The UV/Vis spectra for the polymers are shown in Figure 7. Bandgaps were
ꢁ
ꢂ
calculated with the equation ꢀ = ꢃ , using the sharp absorption onset as the
wavelength (Table 4). Polymer D was opaque to the eye, and therefore does not
have an approximately zeroed baseline like the rest of the transparent polymer
films. The bandgap is directly related to the maximum energy density of a
dielectric material; the trend for bandgaps in these materials, in decreasing order,
is D>A>C>B. This perfectly follows the electronegativity trend with respect to the
central heteroatom in the difuran monomers, indicating that electron-withdrawing
species increase the bandgap.

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1
A
B
C
D
0
0
.75
0
.5
.25
0
2
00
300
400
500
600
700
800
Wavelength, nm
Figure 7 – UV/Vis spectra for the synthesized polymers.
Polymer Absorption Onset,
nm
Band gap, eV
A
B
C
D
520
579
555
477
2.39
2.15
2.24
2.61
Table 4 – Absorbance onset wavelengths and optically determined band gaps for
the synthesized polymers.
Conclusions
Our research group is searching for materials with high dielectric constants by
exploring the chemical space synthetically, with assistance from computational
and processing teams. The polymers described in this study are among others in
a broad library of materials with polar groups incorporated into the polymer
backbone. The dielectric data of these polymers will be fed into a quantitative
structure-activity relationship (QSAR) with the hopes of finding computational
descriptors that accurately predict dielectric properties. Dielectric constants for
the synthesized polysulfones ranged between 4.96-5.98, with dielectric loss

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tangents ranging from 0.4-0.9% at 1kHz and room temperature. Bandgaps for
the polymers ranged from 2.15 – 2.61 eV. Our dielectric constants were at least
double that of BOPP, the current industrial standard, which has a dielectric
constant of ~2.2 across a broad frequency range. To our knowledge, this is the
first time divinylsulfone has been utilized in a DA polymerization, and we intend
to explore other systems using this dienophile. We believe the synergy of
synthetic and computational work has the potential to move science research
away from its traditionally Edisonian approach towards experiment design, which
would help to save money, resources, and the environment in the process.
Acknowledgements
We would like to acknowledge Ms. JoAnne Ronzello of the UConn Electrical
Insulation Research Center for conducting the dielectric measurements. We’re
also grateful to our funding sources, including the Office of Naval Research
through the Multi-University Research Initiative (MURI) for work in the laboratory
of GAS and the Harold S. Schwenk Sr. Distinguished Chair in Chemistry for
work in the laboratory of RRB.
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33
Gilchrist, T.L.; Storr, R.C. Organic Reactions and Orbital Symmetry. Cambridge
University Press, Cambridge. 1979, pg. 94

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-
-
The synthesis and characterization of Diels-Alder polysulfones is described
The syntheses were evaluated a priori using computational HOMO/LUMO data
-
1
Dielectric constants range from 4.96-5.98, loss tangents range from 0.4-0.9% at
kHz
-
2
Bandgaps were determined with UV/Vis spectroscopy, and range between
.15-2.61 eV
-
The retro Diels-Alder temperature was determined with 3 analytical methods

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Diels-Alder Polysulfones as Dielectric Materials: Computational Guidance
&
Synthesis
§,‡
§
§
§,‡,*
Robert G. Lorenzini , Jordan A. Greco , Robert R. Birge , Gregory A. Sotzing
§
‡
Department of Chemistry & The Polymer Program, University of Connecticut,
55 North Eagleville Road, Storrs, CT 06269, United States
Supporting Information
0
.15
0.1
RT
5
7
1
0C
5C
0
.05
0
00C
1
0
100
1000
10000
Frequency, Hz
Dielectric spectrum (left) and dielectric loss tangent (right) of polymer A.
0
.3
.25
.2
.15
.1
0
0
RT
0
5
7
1
0C
5C
0
00C
0
.05
0
1
0
100
1000
10000
Frequency, Hz
Dielectric spectrum (left) and dielectric loss tangent (right) of polymer B.

ACCEPTED MANUSCRIPT
0
.2
0
.15
RT
0
.1
5
7
1
0C
5C
0
.05
0
00C
1
0
100
1000
10000
Frequency, Hz
Dielectric spectrum (left) and dielectric loss tangent (right) of polymer C.
0.05
0.04
0.03
0.02
0.01
0
RT
5
7
1
0C
5C
00C
1
0
100
1000
10000
Frequency, Hz
Dielectric spectrum (left) and dielectric loss tangent (right) of polymer D.